Wix works with javascript on the page and site code tabs, it does not use html.
To add a onClick event handler function to a button in Wix, see these pages here which will explain in more detail.
https://www.wix.com/corvid/reference/$w.ClickableMixin.html#onClick
https://support.wix.com/en/article/corvid-reacting-to-user-actions-using-events
Or simply add a link to the button element itself to set where the button links to,
https://support.wix.com/en/article/adding-a-link-to-a-button-338337
If you are following their own support.
https://help.calendly.com/hc/en-us/articles/223147127
Then you need to be adding it to your Wix page in a html iFrame.
https://support.wix.com/en/article/using-iframes-to-display-visible-content-on-your-site
Wix does not provide support for third party code as stated on the above linked page. If you require more help with this you should be going through Calendly own support.
When clicking the button it shows this for me below, although I did have to turn off my browsers third party extensions for adblocker and popups etc for it to show correctly.
Third party extensions in browsers can affect Wix sites by blocking certain parts of it or certain elements for example, so unless you turn the extensions off and reload the page, or whitelist your sites through the third party extensions, then it won’t work properly.
https://support.wix.com/en/article/disabling-your-pop-up-blocker